A polymer film is often used, for example, as a protective film of a polarizing filter of a liquid crystal display for protecting the polarized film, a wide view film and the like, since having merits in high transparency, high workability, handling easiness, smallness, possibility of miniaturization and the like. Especially, a cellulose acylate film produced by using a cellulose acylate as a polymer is excellent in low cost of raw materials, optical properties (such as wide view angle, high transmittance and the like) and reasonable cost of raw materials. Therefore, attention is focused on the cellulose acylate film with high functions that is produced at low cost.
In order to produce the polymer film, especially the cellulose acylate film, a solution casting method is mainly used. In the solution casting method, a dope is prepared by mixing a polymer (such as cellulose acylate) as the raw material of the film, an organic solvent and the like, and cast thereafter onto a running support to form thereon a casting film containing the solvent. Then the casting film is peeled as a wet film from the support. The wet film, which contains the solvent, is dried to the polymer film.
Generally, plasticizer and an UV absorbing agent and the like are added as additive in the polymer film in order to keep the function of the UV absorbance and increase the brittle strength. However, almost of the additive compounds are volatile compounds having low molecular weights. Therefore, when the casting film and the wet film are dried, not only the organic solvent but also the volatile compounds of the additive evaporate. In the produced film, the effects of the additive become lower. Note that the volatile compound is added as additive which easily evaporates as described above in the film production.
Further, if the film contains the volatile compounds, the volatile compounds deposit on the film surface during storage or in use. Thus the bleeding-out phenomenon occur, which causes the pollution of the film surface and the like. For example, if the polymer film is used as the protective film for the polarizing filter, the adhesiveness of the polarizing filter to the polymer film becomes lower. Therefore, in order to prevent the bleeding-out phenomenon, the Japanese Patent Laid-Open Publication No. 2001-054936 teaches a polymer film and a production method thereof in which surface layers sandwich a main layer, while the content of the plasticizer and the UV absorbing agent is lower in the surface layers than the main layer.
However, the additive (such as the volatile compound and the like), the polymer and the organic solvents are hardly mixed, since there is a difference in compatibility among these compounds. Therefore, it is hard to prepare the dope uniformly. The stirring time is made longer at high rotation speed of a stirrer, which causes the long production time and the high cost. In order to obtain the uniform dope effectively and efficiently, for example, the Japanese Patent Laid-Open Publication No. 2003-053752 teaches a production method of a dope. In this method, the primary dope containing the polymer and the organic solvent is prepared, and then the additive is added to the dope through a nozzle while a circular outlet of the nozzle is disposed in a center of inside of a pipe in which the dope containing the polymer and the organic solvent flows. After the adding, the dope and the additive are mixed by an inline mixer. Further, the conditions of adding and heating, and the pressure of the dope are controlled.
However, in the publication No. 2001-054936, there still remains a possibility of the bleeding-out phenomenon of the plasticizer and the UV absorbing agent that are contained in the surface layers. Further, in the publication No. 2003-053752, the productivity is not high since it takes long time for the additive to spread from a center to an inner face on the tube. If the inline mixer having many elements is used, the productivity becomes higher. However, in this case, the cost becomes higher and the apparatus becomes larger. Further, in the method of adding the additive through the outlet of the nozzle, the additive is not added uniformly, and the uniform dope is hardly obtained.